[Taqwus]

Ease of access to blood becomes a differentiating factor between nations, rather than the fairly odd case of early patchlevels of Dom 2 where any nation with some scouts and some income headroom could bloodhunt to get SDRs, and then use the SDRs to accelerate this (since they did help even non-B mages for a while), in addition to the ubiquitous sorcery or unrestricted randoms.

There's no reason that blood magic should be any easier to achieve regardless of nation than, say, fire-immune infantry or flying assassins, especially when there are nations for whom blood magic is intended to be a major strength.

Might also be noted that blood magic is associated with some very nice national summons (Mandahas, the Tlaloques, the Devatas for instance) and also received some unresistable literally-go-to-Hell combat spells. Also, unlike gems, you have a lot of freedom in deciding where you gather them instead of being reliant on protecting possibly inconvenient locations; if you need to pull back, any surviving blood hunters can fall back, and your opponent does not get your blood income like he would from a magical site.

Regarding SDR, it's only 10 blood slaves paying full retail, and IIRC it acts as an effective +1 for a blood hunter that's already at B1 or higher, so if the hunter is a dedicated hunter it's a bargain compared to the cheapest possible empowerment of B1-B2 at 30 gems. It's cheaper than the Athame, Brazen Vessels and Armor of Souls too.

Magic sites rarely contribute blood slaves; there's the Damned Merchant (+1/turn) and some national capitals (Mictlan's, for instance), but bloodhunting (with B mages...) or wishing are far, far faster. That's not to say that there aren't some extremely interesting blood sites (some giving large summoning bonuses, some 'blood mages may enter to summon some type of devil or demon' sites), but they're pretty rare.


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Edit (addition):

It also occurs to me that blood magic provides the simplest way to deal with aging -- Boots of Youth. Non-blood nations need Elixirs of Life, which are a bit more complicated to make and take a miscellaneous slot -- arguably more important than a foot slot for mages now that quickness no longer accelerates casting... or Gift of Health, which can only be used by one nation at a time and can be dispelled.